Electrolyte for aluminum production and method of preparing same



Patented Aug. 14, 1923.

UNITED STATES PATENT OFFICE.

ALADAR PACZ, OF CLEVELAND HEIGHTS, OHIO ELECTROLYTE FOB ALUMINUMPRODUCTION AND METHOD OF PREPARING SAME.

No Drawing.

To all whom. it may concern.-

Be it known that I, ALADAR Pacz, a citizen of the United States,residing at Cleveland Heights, in the county of (uyahoga and State ofOhio, have invented a certain new and useful lmprovement in Electrolytesfor Aluminum Production and Methods of Preparing Same, of which thefollowing is a full, clear, and exact description.

This invention relates to the production of metallic aluminum andespecially to the composition, source and nature of the electrolyteemployed in such production. According to all commercial practices,aluminum is obtained by the electrolysis of a so lution of alumina infused cryolite. Cryolite has been employed heretofore because of thefact that it is anhydrous, is lighter than aluminum when molten and thusshields the same from the atmosphere, fuses at a fairly low temperature,and dissolves alumina with much ease. The melting point of aluminum is657 (1, and that of cryolite is about 1000 C. but the latter is reducedto 915 by the presence of 5% of alumina, while the addition of certainother substances such as calcium fluoride and aluminum fluoride willreduce the melting point to about 800 C. It is desirable to perform theelectrolysis at the lowest possible temperature in order to minimizeheat loss and current loss. Theoretically, 42 grams of aluminum perkilowatt hour should be reduced by the electrolysis of alumina butpractically the best figures obtainable for aluminum production showabout 30 grams per kilowatt hour .which corresponds to an efficiency of71%.

These is a difference of opinion as to what occurs in the bath duringelectrolysis, some asserting that the alumina is directly decomposedinto its elements and others holding that the aluminum fluoridecomponent of the cryolite is decomposed by the current and continuouslyregenerated by a secondary reaction of the dissociated fluorine upon theadded alumina. The composition of the bath employed is kept Secret bymost producers but always heretofore has comprised cryolitc with othermaterials to reduce the melting point to which pure aluinina iscontinuously added. heorctically, the best. melting: point reducer isaluminum fluoride as set forth by the most authoritative patentecs andwriters, this being in Application filed March 18. i920. Serial No.366,867.

conformity with the hypothesis that the aluminum fluoride is the primaryactive material; but this substance has never come int wide use owing toits high cost, as it is not a natural product nor even a by-product ofany operation hitherto conducted. The melting point reducer ordinarilyused is calcium fluoride, which however, is of no value in the reactionbut is to some extent ficial cryolite could he produced at a cost tocompete with the natural product.

The objects of my invention are the provision of a new and improvedelectrolyte for'the production of metallic aluminum, independently ofnatural cryolite; the provision of an electrolyte for this purpose whichshall have a much lower melting point and a much higher currenteliiciency; the provision of a new and improved electrolyte for thispurpose which shall be made from substances at present a wasteby-prodact of large industries, and which supply is capable of unlimitedexpansion; the provision of a new and improved method for the productionof silicon especial] in company with other metals such as a uminum; theprovision of a new and improved method for reducing and obtaining othermetals in combination with aluminum such as boron, titanium, zirconium,thorium, beryllium, and rare earth metals, vanadium, columbium,tantalum, iron, manganese, nickel. chromium, cobalt, copper, uranium;the provision of a process for the produc tion of artificial cryolite ata cost to compete with the natural product; while further objects andadvantages of my invention will become apparent as the descriptionproreeds.

My improved electrolyte is prepared by the action of aluminum upon thedouble fluoride of sodium and a metal whose fluoride is reduced byaluminum under the onditions which surround the reaction as actuallyperformed. The substances named are mixed in finely divided condition,the amount of aluminum being such as to displace the other metal fromthe double fluoride. In order to hold the ingredients in their properlymixed condition. prevent segregation, and enable their simultaneoussulunergence in the molten bath preterably agglomeratc The. mlxture intolumps, which may take the form of bricks, briquettes, blocks chunks, orpellets and be accomplished either by the use of a suitable binder, suchas sodium silicate or even by pressure alone in some instances. Thereaction is effected by adding these lumps to a bath of molten metalwhose temperature is at least as high as that of fused aluminum. I

The cheapest and most readily available double fluoride i's that. knownas sodium silico-fiuoride (Na SiF which isproduced in great quantites inthe commercial manufacture of certain fertilizers known as superphosphates and which is very largely wasted at the present time. itsonly commercial use which is at all extensive being in connection withthe enameling of sheet metal which utilizes only a very small part ofthe output. I take a quantity of this substance in the powdered form (itis in this form that it is normally produced) and mix it Int lmatelywith powdered or blown metall c aluminum in the proportion ofapproximately five parts of the former to one of the latter. In order tohold the two ingredients in intimate relatiorl with each other and toprevent segregation which might easily occur as the result of theirdifferent specific gravities, I preferably briquette or agglomerate themixture into lumps by means of a suitable binder such as a dilutedsolution of sodium silicate.

This material is then introduced into a bath of pure molten aluminum,maintained at any temperature above its fusion point, whereupon thefollowing reaction o-curs sua sir cuzasi+eNar+4iuF The reaction isslightly but not strongly exothermic and occurs very quickly. the bathbeing preferably stirred continuously while it is taking place. Thesilicon released by the reaction becomes alloyed with the aluminum ofthe bath while the remaining ingredients rise to the top of the bath inthe form of a clear molten slag. The alloy of aluminum and silicon thusproduced when treated as described and claimed in my earlier applicationfiled February 13. 1920, Serial No. 35 .555. exhibits features of thevery highest commercial value. so that the silicon .illoy constitutes avery valuable hy-product. The slag which is pl fultltefl by thisreaction is easily removed either by ladling or pouring and forms amost'perfeet electrolyte for the production of aluminum. consisting asitdoes essentially of artificial cryolite mixed with an excess ofaluminum fluoride. the'latter constituting about i-ltl of the whole.This is not to assert that the ingredients of this slag or batlrarccombined together in the way that natural cryolite is combined. This isa theoretical point and its discussion is not material at the presenttime. \Vhether the bath be considered as a mixture of cryolite andaluminum fluoride or whether it be considered as consisting of someother combL nation of the various ingredients the fact. remains that itsmelting point is more than 200 less than that of any natural cryolitebath ever before produced and at least as low as that of pure aluminum;its electrical conductivity is at least as great, its content of activeingredients is greater than any commercially employed electrolyte. andthe solubility of alumina therein is at least as great. As a result ofthese advantages I find it possible to produce aluminum at aconsiderably higher current efficiency than is now obtained in anypractical establishment. and higher than any obtained under the mostperfect laboratory conditions of which I am aware. In every respect thiselectrolyte is superior to the best natural cryolite.

As to the cost of this bath. the use of 564 pounds of sodiumsilico-fiuoride with 108 pounds of aluminum (these being the strictlytheoretical proportions) will produce 84 pounds of silicon (alloyed withthe bath) and 588 pounds of the improved electrolyte and nasmuch as thebest available figures provide for a wastage of the crvolite bath at therate of pound of the bath for each pound of aluminum produced. thisamount of electrolyte will serve for the production of nearly 6000pounds of aluminum. Consider ng the fact that the sodium silicofluorideis practically a Waste product and that the electrical efficiency is somuch greater'and the heat loss so much less with my improvedelectrolyte. this more than of?- sets the cost of the aluminumoriginally em ployed; in addition to which the value of thealloy-silicon produced largely offsets the cost of the aluminumemployed. It

In fact my observation is that the rate of waste of the bath is muchless with my improved electrolyte than with the baths here tofore usedand I believe that the absence of su h foreign substances as calcium islargely responsible for this longer life.

Sodium Silico-fiuoride is not the only source of my improved electrolyteand I have only instanced the same as being )6:- culiarly available.Nearly every don le fluoride of sodium and some other metal can beemployed, for example boron, beryllium. scandium, ytterbium, lanthanum,

titanium, zirconium, cerium, thorium, vanadium, niobium, tantalum,chromium, manganese. iron, cobalt, nickel, copper. or uranium. shownabove. the silicon aluminum replacing the metal in question whichappears alloyed with the aluminum bath, and I apprehend that aconsiderable source of this electrolyte will be as a by-product in someof these reactions inasmuch as this constitutes an easier mode ofreducing many of those metals than any other mode now known. The factthat the metals are obtained in admixture with another metalconstitutes-no serious disadvantage since those alloys are frequentlyvaluable for themselves and the metals can be separated therefrom bydissolving out one substance with some reagent that does not attack theother substance. For example aluminum will dissolve in both hydrochloricacid and sodium hydroxide and almost no other metal will succumb to bothof them.

In fact I do not restrict myself to the use of an aluminum bath sincethe same reactlon occurs when the mixture of double fluoride andaluminum powder is introduced into any other molten metal and in thisway numerous very valuable alloys are made but always with theproduction of this peculiar and valuable electrolyte as a slag.

In this specification and claims I have included silicon under the termmetal inasmuch as its properties, both when al- 10 ed with aluminum andwhen combined with other metals in difl'erent chemical compounds areclearly those of a metal.

It will be understood that I do not restrict myself to an electrolyte ofthe exact composition described since the same can be either diluted orfortified by the addition of other substances, but I claim and assert asmy invention all electrolytic baths wherein any part is produced inaccordance with the process here described, limiting myself in no wisewhatsoever except as specifically set forth in my claims.

Having thus described my invention. what I claim is:

1. Method for the production of electrolyte for the anhydrous reductionof metals which contains the step of reacting with aluminum upon adouble fluoride of sodium and a second non-aluminous metal at a.

temperature above the melting point of In all cases the reaction isasaluminum upon sodium fluo-silicate at a temperature above the meltingpoint of aluminum and in the presence of an excess of aluminum.

4. Method for the production of electrolyte for the anhydrous reductionof metals which contains the step of introducing into a bath of fusedaluminum an intimate mixture of aluminum powder and sodium fluosilicate.

' 5. Method for the production of electrolyte for the anhydrousreduction of metals which contains the step of introducing into a bathof fused aluminum an intimate mixture of aluminum powder and a doublefluoride of sodium together with yet another metal.

6. Method for the production of electrolyte for the anhydrous reductionof metals which contains the step of introducing into a bath of fusedmetal an intimate mixture of aluminum powder and a double fluoride ofanother metal wit sodium.

7. Method for the production of artificial cryolite which contains thesteps of introducing the double sodium fluoride of an other metalintimately mixed with the amount of powdered aluminum necessary toreplace such other metal, into a bath of molten metal whose temperatureis at least as high as the melting point of aluminum, and subsequentlyseparating the slag from the metallic constituents.

8. Composition of matter for the production of artificial cryolitecomprising the double fluoride of sodium and a. metal intimately mixedwith the amount of powdered aluminum required to displace such othermetal.

9. A composition of matter for the purpose described comprising thedouble fluoride of sodium and some metal whose halogen compound isreduced by aluminum. mixed with the amount of powdered aluminum todisplace such metal and agglomerated into lump form.

10. A composition of matter for the purpose described comprising sodiumsilicofluoride mixed with a suflicient amount of powdered aluminum to relace the silicon content and agglomerated into lump form.

11. A-composition of matter for the purpose described comprising anintimate mixture of sodium fluoride, aluminum powder, and the fluorideof some metal which is replaoeable by aluminum, the aluminum bemgpresent in sufficient quantity to replace such other metal and thesodium fluoride being present in such quantity as to form with theresulting aluminum fluoride a composition which is substantiallycryolite mixed with an excess of aluminum fluoride.

12. Method for the production of electrolyte for the anhydrous reductionof aluminum which contains the step of subjecting a solid mixture ofsodium fluoride and the fluoride of some metal to the action of powderedaluminum at. a temperature above the melting point of aluminum, unde airex- 5 eluding conditions, said metal being of a nature whose fluorinecompound is reduced by aluminum under the conditions stated.

13. Process of making electrolyte for aluminum reduction which containsthe step of decomposing sodium silica-fluoride bv 10 metallic aluminumin the presence of heat. In testimony whereof, I hereunto aflix mysignature.

ALADAR PACZ.

